The present invention relates generally to energy storage and, more particularly, to methods and apparatus for storing electrical energy (e.g., in a vehicle), in a manner that maximizes the useful life of the storage capacitors.
In order to store electrical energy (e.g., in electric vehicles, hybrid vehicles, etc.), capacitor storage device are presently used as well as rechargeable electrochemical storage devices. The selected storage medium depends on the respective requirements of the energy storage. Electrochemical storage devices such as nickel cadmium batteries, nickel metal hydride batteries, lead acid batteries or lithium batteries are particularly suitable as long-term storage devices due to their high specific storage capacity (energy density). xe2x80x9cLong-term storage devicesxe2x80x9d are to be understood as energy storage devices that can store large amounts of energy but in comparison have a relatively low output. For uses that require a high output over short periods, capacitors are preferably used as xe2x80x9cshort-term storage devicesxe2x80x9d such as electrolyte or ceramic capacitors. Since the charge carriers are stored in a purely physical matter (electrostatically) in the capacitors, the stored energy can be released within 10xe2x88x923 to 10xe2x88x925 s. At up to 100,000 W/kg, the power density is hence approximately 100 times greater than electrochemical storage media. As a trade-off, capacitor storage devices have substantially lower specific storage capacity than electrochemical storage devices.
For many applications, it is especially useful to combine an electrochemical storage device that has a high energy density with a capacitor storage device that has a high power density. DE 198 40 819 discloses, for example, the use of high power capacitors for a short-term storage device in a vehicle that delivers all or part of the starting energy to the starter upon starting the internal combustion engine. The short-term storage device is charged as a preparation for starting by drawing power from an electrochemical battery. In this manner, the battery can quickly release energy stored over a long period to the starter and start the internal combustion engine even when the power offered by the battery is insufficient.
For a few years, a new generation of capacitors has been available that are termed xe2x80x9celectrochemical capacitors,xe2x80x9d xe2x80x9cdouble-layer capacitors,xe2x80x9d or xe2x80x9csuper capacitors.xe2x80x9d Such capacitors include two porous electrodes, usually made of active charcoal, with a very high internal surface of up to 10,000 m2/g. The electrodes are saturated with an electrolyte. When a voltage is applied, the ions of the electrolyte collect on the electrodes. An electrical double-layer or boundary layer forms that includes a layer of electron charges in the solid, and a neighboring layer of ion charges in the electrolyte. The water bound to the ions, for example, functions as a dielectric so that the spacing of the xe2x80x9ccapacitor platesxe2x80x9d equals the diameter of the water molecules. Given this extremely small spacing between the charge layers and large surface of the active charcoal electrodes, we obtain extremely high capacitances of approximately 200 F/g (see L. F. Trueb, P. Rxc3xcetschi: xe2x80x9cBatterien und Akumulatorenxe2x80x94Mobile Energiequellen fxc3xcr heute und morgenxe2x80x9d (Batteries and Accumulatorsxe2x80x94Mobile Energy Sources for Today and Tomorrow), Springer-Verlag: Berlin 1998). Double-layer capacitors are commercially available under the name of xe2x80x9cUltraCapxe2x80x9d by Siemens Matsushita, or xe2x80x9cGoldCapxe2x80x9d by Panasonic.
The capacitance of the double-layer capacitor can be further increased when a fast, reversible redox reaction also occurs at the electrode surface. In such a case, the double-layer capacitance is added to the chemical or Faraday or pseudocapacitance. For the electrodes, ruthenium or iridium oxide can be used, for example, that changes back and forth very easily and reversibly between oxidation states +3 to +4 during charging and discharging. Thanks to the redox reaction, the charge of such a metal oxide capacitor (also termed a Faraday capacitor) stored on the electrode surface is extremely high. The charging and discharging procedures are slower than with electrostatic capacitors, however they are much faster than purely electrochemical storage devices.
Double-layer and Faraday capacitors have a much higher energy density than conventional capacitors and are, therefore, particularly well suited to use as short-term storage devices. The charging and discharging time is approximately 0.3 to 30 seconds, and the energy can be stored over a few days to weeks (see Thomas Dietrich, xe2x80x9cKondensator-Power fur innovative Applikationen,xe2x80x9d (Capacitor Power for Innovative Applications), Elektronik, Vol. 9/99, p. 72-77).
The useful life of electrochemical capacitors is usually cited as being approximately 10 years. In practice, however, this life is rarely attained. The electrochemical capacitors can only be operated at relatively low voltages. At higher voltages, electrochemical decomposition (electrolysis) of the electrolyte starts, which leads to the irreversible destruction of the capacitor. With capacitors that have an aqueous electrolyte, H2 and O2 arise, whereas, for example, carbon dioxide arises in the electrolysis of organic electrolytes. The gas pressure that this causes expands the housing and increases the internal resistance of the capacitor. When used in a vehicle, the life of a double-layer or Faraday capacitor is therefore limited to approximately one year.